In the past years, plastics have become commercially vital materials used in a wide range of applications. Particularly, plastics may be applied as transparent articles, as translucent articles, or to opaque surfaces to provide texture, shine, and durability. Generally, these applications require a high degree of scratch- and mar-resistance in addition to other required properties, for example, impact strength, tensile strength, or elongation.
To date, no plastic exists which can be made to possess all desirable properties in the desirable proportions. Therefore, a laminations and coatings technology has developed which allows the joining of plastic materials and thereby a joining of their desirable properties. For example, the laminations and coatings technology may be used to join a substrate possessing high impact resistance, tensile strength, non-opacity, and elongation resistance, such as for example, polycarbonate, with an outer surface layer possessing high scratch-resistance, mar-resistance, and low susceptibility to attack by solvents, such as for example, cross-linked polyacrylates, to produce a single article possessing the structural strength of the polycarbonates and the resistance to surface attack of the cross-linked polyacrylates.
Many prior art laminations and coatings efforts have been unsuccessful for a variety of reasons. Some are unsuccessful due to the incompatibility of the laminae or coating materials with the plastic substrate. Some may be judged unsuccessful by their failure to impart the required degree of a particular desirable property to the substrate. Others may be replaced by the development of new and more desirable coatings or laminates.
The replacement of known coatings by a new development in the art is particularly likely in the area of providing scratch- and mar-resistance to a plastic substrate. Scratch- and mar-resistance are always desirable in greater degree for the uses in which they are necessary, for example, to increase the useful life of a transparent plastic article, the decorative qualities of a translucent plastic article, or the shine of an opaque plastic surface.
The prior art scratch- and mar-resistant coatings for plastic substrates, such as, the polycarbonates have included organopolysiloxanes, U.S. Pat. No. 3,707,397; polyester-melamines or acrylic-melamines, U.S. Pat. No. 3,843,390; and allyl resins, U.S. Pat. No. 2,332,461. These types of prior art coatings generally involve a thermal curing process which is expensive and creates problems with warpage.
U.S. Pat. No. 3,968,305 describes a synthetic shaped article having a mar-resistant polymer surface layer integrated with the polymer surface body, said polymer surface layer consisting essentially of, in polymerized form, (a) 20 to 100 weight percent of a compound having a total of at least three acryloxy and/or methacryloxy groups linked with a straight chain aliphatic hydrocarbon residue having not more than 20 carbon atoms, (b) 0 to 80 weight percent of at least one copolymerizable mono- or diethylenically unsaturated compound. This type of a surface layer suffers from the fact that it generally has poor durability of adhesion after prolonged exposure to weathering.
U.S. Pat. No. 3,968,309 describes a molded article of plastic having on its surface a cured film of a coating material comprising at least 30% by weight of at least one polyfunctional compound selected from the group consisting of polymethacryloxy compounds having a molecular weight of 250 to 800 and containing at least three acryloyloxy groups in the molecule. This patent also teaches that the coating must contain from 0.01% to 5% by weight of a fluorine-containing surfactant in order for the coated article to be acceptable.
Although scratch- and mar-resistance are enhanced, an even greater degree of such protection is desirable.
U.S. Pat. No. 4,198,465 discloses a plastic substrate, preferably a polycarbonate, having adhered thereto a durably adherent mar-, scratch-, and chemical-resistant UV cured coating comprised of the photo-reaction products of a photoinitiator, resorcinol monobenzoate, and a mixture of polyfunctional acrylate monomers. While the coating of this disclosure provides scratch-, mar-, and abrasion-resistance, the useful life of many plastic articles can be increased with more such protection.
U.S. Pat. No. 4,319,811 discloses an invention relating to a radiation curable coating which provides abrasion- and chemical-solvent resistance. The coating is a comonomer having a first monomer selected from the group consisting of the triacrylates and tetracrylates mixed with a second monomer having an N-vinyl imido group, preferably an N-vinyl lactam, such as N-vinyl pyrrolidone or N-vinyl caprolactam. A photoinitiator, for example (p-phenoxy)dichloroacetophenone or dimethoxyphenyl acetophenone, is included in the photocurable mixture. A greater degree of scratch- and mar-resistance is desirable than is provided by this coating.
It is now been found that a photocurable coating composition containing a comonomer of certain specific polyfunctional acrylates and a second comonomer of certain substituted or unsubstituted acrylonitriles provides superior and durable UV cured coatings for plastic substrates. Thus, the present invention provides comonomer based UV cured coatings for application to transparent plastic articles, transluscent plastic articles, or opaque plastic surfaces, which adhere tenaciously and durably to the substrate, are compatible with the substrate, are mar-, scratch-, and solvent-resistant, and are not deleteriously affected after prolonged exposure to weathering.